This invention relates to manhole covers for access to enclosed spaces such as tanks, conduits and storage areas and more particularly relates to mechanisms to assist in balancing the weight of manhole covers.
Such balancing mechanisms have been known, as for example are disclosed in U.S. Pat. No. 4,137,669, incorporated herein by reference.
Typically, the weight of the manhole cover is at least partially balanced by a spring mechanism, e.g., as shown in U.S. Pat. No. 4,137,669. Such balancing mechanisms have been very beneficial in reducing the amount of force required to lift and retain the manhole cover in an open or partially open position and have reduced risk of injury as a result of muscular strain in lifting the cover and have also reduced the probability of injury to person or property as a result of the full unbalanced weight of the cover falling upon a person or property.
Nevertheless, such balancing mechanisms still have certain disadvantages. In particular retaining bolts can be removed from such devices when the cover is closed, e.g., to entirely remove the cover or for maintenance, without first releasing the considerable energy stored in the balancing springs which may result in dangerous and unpredictable release of such energy.
In addition, such mechanisms have had a considerable number of exposed nuts, threads and other parts which tend to collect dirt in the form of dust, grease, chemicals and other contamination and are difficult to clean Such mechanisms have also tended to corrode. These tendencies have made known balancing mechanisms for manhole covers less than desirable for use in food and pharmaceutical industries.